1. Field of the Invention
The present invention relates generally to camera imagers (such as photographic cameras), and, more particularly, to an electronic system and method for compensating for image unsteadiness in camera imagers.
2. Related Art
In using camera imagers to electronically collect sensed images, the length of time during which a patch of object space is imaged onto a corresponding angularly-sized pixel is referred to as the exposure time. The signal level generated by the camera imager is directly proportional to this exposure time. Thus, all other things being equal, longer exposure times produce larger signal levels. Larger signal levels have better pixel-to-pixel signal-to-noise ratios relative to those having shorter exposure times.
However, as exposure time is increased, two problems arise. First, the area of the scene which can be covered or scanned per unit time becomes restricted, sometimes to the point of diminished system utility. Second, the amount of image smear resulting from vehicle perturbations may increase. If this image smear is not compensated for, it may reduce image quality.
Conventional techniques have solved the first problem by using time delay and integration (TDI) detectors. TDI detectors effectively increase available exposure time by accumulating a larger amount of charge while eliminating the penalty of minimizing coverage. This desirable result is obtained because a sensor using a number (N) of TDI stages can scan N times as fast as a corresponding linear detector. These multiple stages enable multiple coverage of the same portion of the scene, resulting in a charge which is proportional to the number of TDI stages used. Thus, a TDI detector provides approximately N times the angular coverage of a corresponding linear detector which utilizes the same total exposure time. The use of TDI detectors, however, does not solve the second problem. Specifically, the use of TDI detectors does not eliminate the effects of image smear (also referred to as image unsteadiness) as a function of total exposure time.
Conventional systems which have been employed to stabilize unsteadiness and avoid image smear use mechanical or optomechanical methods to stabilize the sensor with respect to the object space. In many instances, however, it is difficult to achieve the desired tolerance levels. Subsequently, the exposure time and number of TDI integrators which may be effectively employed are negatively impacted. This can often be the limiting factor on system performance and mission satisfaction. Therefore, an effective and efficient system and method for compensating for image unsteadiness is required.